Power brake boosters of the differential pressure type using engine vacuum are well known in the motor vehicle industry. A brake booster of this type has a diaphragm separating a vacuum chamber open to a source of vacuum, such as created in an engine intake passage, and a working chamber normally provided with vacuum or atmospheric air by a valve apparatus responsive to input brake pedal actuation to provide brake assist force as required through a power piston fixed to the diaphragm and coupled to the booster output.
The valve apparatus may have a vacuum valve seat on the power piston that is engageable with a floating control valve as well as an air valve that also has a seat engageable with the same side of the floating control valve and is coupled to a brake input rod. In the rest position (no brake apply) the air valve is engaged with the floating control valve, and the vacuum valve is slightly separated therefrom to guarantee vacuum on both sides of the diaphragm and thus no brake boost assist. A brake input to the air valve moves the air valve seat axially past the vacuum valve seat, whereupon the floating control valve moves against the vacuum valve seat to shut off the flow of vacuum into the working chamber and allow the air valve seat to disengage and admit air at atmospheric pressure into the working chamber. The increase in air pressure in the working chamber produces a differential pressure across the diaphragm to move it axially; and the power piston move with it to provide an assist force to the brake fluid in the master cylinder. Movement of the power piston allows the floating control valve to move toward the air valve seat; and the level of boost assist is thus matched to the driver's input force. The opening of the air valve is calibrated to provide smooth brake apply in normal and typical driving situations.
Occasionally a driver may wish or need to stop the vehicle as quickly as possible. In such a situation, it might be possible to provide slightly quicker brake application by admitting air to the working chamber of the brake booster at a faster rate. But this faster air admittance should not be provided except in such situations, since it would not produce appropriate braking behavior in normal driving.